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Mars rover's broken wheel is beyond repair

Mission managers have given up hope of fixing a broken wheel on NASA’s Spirit rover and will simply have to drag the wheel on future drives. The glitch means NASA must avoid terrain with loose soil as it maps out a route to a safe winter haven for the rover.

The rover’s right-front wheel stopped turning about two weeks ago – apparently because of a broken circuit in the motor that powers the wheel. The same wheel had experienced a surge in current in 2004 but later returned to normal.

But engineers have lost hope that the wheel can ever recover again. Recent tests at a range of voltage levels failed to produce any movement in the wheel. “It’s just not responding,” says team member Ray Arvidson of Washington University in St. Louis, Missouri, US.

He says the wheel will just drag along as the rover’s five other wheels propel it. The loss of the wheel means engineers must take extra care to avoid terrain with steep slopes and loose soil, which can cause the rover to slip. “It’s just like driving through snow – you want to avoid the snow banks,” Arvidson told New Scientist.

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Fading of the light

Late last week, such slippage caused mission managers to reverse the rovers course and descend several metres from its climb towards a north-facing slope on McCool Hill.

Mission managers are trying to put Spirit on a north-facing slope to maximise the sunlight falling on its solar panels during the Martian winter, which will last for about six months. It is not clear how low the rover’s power supply can go before the rover will die, says principal scientist Steve Squyres of Cornell University in Ithaca, New York, US. But he adds&colon; “The more of a northward tilt we get, the more power we’ll have. The more power we’ll have, the better our chances of survival.”

Rover managers are meeting this week to discuss those strategies and weigh the advantages and disadvantages of three potential winter havens, all within 100 metres of the rover’s current location. The rover began the mission with 900 watt-hours when it landed in January 2004, but the Sun’s low angle means it is now operating on about 330 watt-hours of power.

That means it can drive for no longer than one hour per day, but Arvidson says mission planners should not simply head for the nearest north-facing slope. “We have to plan not only over the next few weeks but the next few months,” he says. “We don’t want to drive to a convenient place and have it be a dead-end for science.”

But he adds that the rover’s wheel failure is not entirely bad for the mission. “It’s exposing the underlying materials as we go – that’s a bonus.”